It is known that conjugated acetylenic monomers, such as diynes, triynes, tetraynes, hexaynes and the like can be polymerized from the state of colorless crystalline solid monomers to colored polymers by thermal activation (U.S. Pat. No. 3,999,946 of Dec. 28, 1976 to Patel et al., column 4, line 13-column 5, line 58). This patent also discloses a photo induced reaction of such acetylenic monomers (column 8, lines 3-13).
The patent also states at column 9, lines 45-50 that bromine solution or vapor preferentially reacts with the unreacted (i.e. monomeric) acetylenic compounds, thereby eliminating the reactive conjugated acetylene groups employed in time/temperature indicator compositions; and so is used to deactivate the indicators, for purposes of "freezing" information so that further exposure to heat no longer produces a color change (see column 9, lines 9-12).